Definition:
The BEST index was designed to be simple to calculate and to provide a long time period ENSO index for research purposes. Nino 3.4 has traditionally been used as a measure of ENSO strength in the tropical Pacific. However, it's use alone neglects explicit atmospheric processes. By adding the SOI or Southern Oscillation Index (the pressure difference between Tahiti and Darwin), these processes are more directly included. In addition, older SST values are at least partially reconstructed and not explicitly measured. By including the SOI, which is better measured historically, the effect of biases in the SST data introduced by the reconstruction technique are reduced. A more detailed explanation how the index was created is available (http://www.esrl.noaa.gov/psd/people/cathy.smith/best/details.html).

Reference:
U.S. Department of Commerce | National Oceanic and Atmospheric Administration Earth System Research Laboratory | Physical Sciences Division

Smith, C.A. and P. Sardeshmukh, 2000, The Effect of ENSO on the Intraseasonal Variance of Surface Temperature in Winter., International J. of Climatology, 20 1543-1557.